The present invention relates to a method of controlling a thickness of a biaxially oriented film in an automatic T-die, and more particularly to a method of controlling and observing a thickness of a biaxially oriented film by accurate setting of corresponding positions on the film to bolts of the T-die.
In prior art, the control in the thickness of the biaxially oriented film was generally carried out as follows. A sheet of a plastic material is extruded from an automatic T-die having a plurality of die bolts for adjustment of a gap of an extrusion port of the automatic T-die. The extruded plastic material sheet is cooled to be solidified in a cast roll unit. The solidified sheet is then stretched or drawn through a sequential biaxial drawing machine which comprises a vertical drawing and a horizontal drawing with a hot air blower to form a film product at the time when the thickness of the film is measured to control the T-die.
The distribution of the thickness of the film product in a direction of the film width is measured by use of a scanning thickness gauge. The measured thickness of the film is compared to a reference thickness previously determined to find deviations in the thickness of the film versus the reference value so as to find a position where the deviation in the thickness of the film is over a predetemined value whereby adjustment of the bolt located at the above position is carried out with reference to the found deviations of the thickness.
It is possible to continuously form the film product having a uniform thickness in the direction of the film width by use of the above method.
The above method of controlling the thickness of the biaxially oriented film has the following disadvantages.
According to the above conventional method of controlling the thickness of the biaxially oriented film, the deviation of the measured thickness of the film from the predetermined reference thickness is found so that the adjustment of the bolt located at the position corresponding, in the film width direction, to the position where the found deviation is over the predetermined value is placed under the feedback control. Actually, however, it was difficult to accurately set the die bolt at the position corresponding to the position on the film. Namely, it is difficult to determine the corresponding position on the film to the position where the die bolt is located. This makes it difficult to actually carry out the required accurate control and observation of the thickness of the film.
The above difficulty in determining the corresponding position on the film to the position where the die bolt is located is caused by the fact that the position corresponding to the position of the bolt is set by regarding the side drawing rate as constant even though the side drawing rate is actually varied in the direction of the film width. Such variation of the side drawing rate in the direction of the film width causes not only the variation in thickness of the film product but also variation in tensile strength of the film product, resulting in deterioration in quality of the film product.